Exhaust gas desulfurizing method and apparatus

ABSTRACT

A method of and an apparatus for removing sulfur oxides (SO2, SO3) contained in combustion gases by using activated carbon. In a series of operation of removing SO2 and SO3 in the exhaust gas by adsorbing them on activated carbon and washing the activated carbon with water, the resultant dilute sulfuric acid (H2SO4) is subjected to heat exchange with part of the exhaust gas, whereby the dilute sulfuric acid is concentrated and the temperature of the exhaust gas is lowered. The cooled exhaust gas is mixed with the main flow of uncooled exhaust gas to adjust the temperature of the whole exhaust gas to a level optimum for enabling the activated carbon to exhibit its highest adsorbing activity, and then the exhaust gas is contacted with the activated carbon, whereby the sulfuric oxides and other harmful gases contained in said exhaust gas are removed and the exhaust gas is released into the atmosphere at such a temperature as will not cause smoke pollution.

United States Patent Tamura et aI.

[ Nov. 20, W73

[ EXHAUST GAS DESULFURIZING METHOD AND APPARATUS US. Cl. 55/73, 55/74Int. Cl. 801d 53/04 Field of Search 55/48, 73, 84, 179,

55/186, 74; 23/2 R, 25 Q, 168, 178 S References Cited UNITED STATESPATENTS 7 3,294,487 12/1966 Pauling 23/168 3,486,852 12/1969 Tamura etal. 23/168 FOREIGN PATENTS OR APPLICATIONS 709,680 5/ I965 Canada 55/84Primary ExaminerCharles N. Hart Attorney-Craig, Antonelli & Hill [57]ABSTRACT A method of and an apparatus for removing sulfur oxides (S0 S0contained in combustion gases by using activated carbon. In a series ofoperation of removing S0 and S0 in the exhaust gas by adsorbing them onactivated carbon and washing the activated carbon with water, theresultant dilute sulfuric acid (H 80 is subjected to heat exchange withpart of the exhaust gas, whereby the dilute sulfuric acid isconcentrated and the temperature of the exhaust gas is lowered. Thecooled exhaust gas is mixed with the main flow of uncooled exhaust gasto adjust the temperature of the whole exhaust gas to a level optimumfor enabling the activated carbon to exhibit its highest adsorbingactivity, and then the exhaust gas is contacted with the activatedcarbon, whereby the sulfuric oxides and other harmful gases contained insaid exhaust gas are removed and the exhaust gas is released into theatmosphere at such a temperature as will not cause smoke pollution.

5 Claims, 1 Drawing Figure L 7 5L ,J P

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PMENTEBmvzo ms EXHAUST GAS DESULFURIZING METHOD AND APPARATUS BACKGROUNDOF THE INVENTION 1. Field of the Invention This invention relates to amethod of removing sulfur components from the combustion gases exhaustedfrom boilers or the like utilizing heavy oil as fuel, and to anapparatus for practicing the method. More specifically, the inventionrelates to improvements in the exhaust gas desulfurizing method andapparatus utilizing activated carbon. Namely, in a series of operationof removing S and S0 in the exhaust gas by absorbing them on activatedcarbon and washing the activated carbon with water, the resultant dilutesulfuric acid is subjected to heat exchange with part of the exhaustgas, whereby the dilute sulfuric'acid is concentrated and thetemperature of the exhaust gas is lowered, and the cooled exhaust gas ismixed with the main flow of uncooled exhaust gas to adjust thetemperature of the whole exhaust gas to a level optimum for enabling theactivated carbon to exhibit its highest adsorbing activity, and then theexhaust gas is contacted with the activated carbon, whereby the sulfuricoxides and other harmful gases contained in said exhaust gas are removedand the exhaust gas is released into the atmosphere at such atemperature as will not cause smoke pollution.

2. Description of the Prior Art A conventional exhaust gas desulfurizingapparatus of the type wherein sulfur oxides are adsorbed on activatedcarbon and desorbed therefrom by washing the activated carbon withwater, is composed mainly of several activated carbon packed towers andseveral washing water tanks. For instance, an SO -containing exhaust gasgenerated from a combustion apparatus, such as a boiler, is led into aflue, pressurized by a blower and led through another flue into theactivated carbon packed towers wherein S0 is removed by the activatedcarbon, and then released into the atmosphere from a chimney. Theactivated carbon in the towers can be regenerated by washing it withwater to desorb the S0 therefrom in the form of H 80 For washing theactivated carbon with water, the water in the water tank is sprayed fromabove the activated carbon through a pipe by means of a pump, and theresultant dilute sulfuric acid drained from the bottom of said watertank is led into the other water tank. The dilute sulfuric acid thusaccumulated in the water tank is concentrated by heat from a separatedheat source and col lected as sulfuric acid of high concentration. Thefirstmentioned water tank is continuously supplied with industrial waterin an amount corresponding to the amount of the dilute sulfuric aciddischarged from said water tank and the amount of water retained by theactivated carbon.

SUMMARY OF THE INVENTION The object of the present invention is toimprove the conventional method and apparatus described above andthereby to enhance the thermal efficiency and economy of the apparatus.Namely, according to the present invention, instead of using the energyfrom the external heat source for heat-concentrating the dilute sulfuricacid resulting from washing the sO -adsorbed activated carbon withwater, the sensible heat of part of the exhaust gas, branched from themain exhaust gas flow, is utilized to concentrate the dilute sulfuricacid to collect it as sulfuric acid of high concentration, andfurthermore said branched exhaust gas cooled by heat exchange with thedilute sulfuric acid is again joined with the main exhaust gas flow toadjust the temperature of the whole exhaust gas at the inlets of theactivated carbon packed towers to about C., whereby the power ofadsorption and the useful life of the activated carbon are improved. Atthe same time, the temperature of the treated exhaust gas, released fromthe chimney into the atmosphere, is maintained above 100C., facilitatingthe spreading of the gas into the atmosphere and thereby preventingatmospheric pollution.

According to the present invention there is provided an exhaust gasdesulfurizing apparatus for removing sulfur oxides from the exhaust gasby using activated carbon, comprising several activated carbon packedtowers, a main flue for leading the exhaust gas from a combustionapparatus into said activated carbon packed towers, a flue branched fromsaid main flue, means for washing the activated carbon in said towerswith water to desorb the sulfur oxides therefrom, means forconcentrating the resultant dilute sulfuric acid from said towers byheat exchange with part of the exhaust gas supplied thereto through saidbranched flue, and a flue for leading the exhaust gas from said dilutesulfuric acid concentrating means into said main flue to mix the samewith the main exhaust gas flow before said main exhaust gas flow isadmitted into said activated carbon packed towers.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a block diagram showingan embodiment of the exhaust gas desulfurizing apparatus according tothe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of theexhaust gas desulfurizing apparatus of the invention will be describedin detail with reference to the accompanying drawing.

An SO -containing exhaust gas from a boiler 1 flowing in a flue 2 ispressurized by a blower 3 and flows into a main flue 4. The main flue 4is branched into flues 5 and 6. In the embodiment shown, thedesulfurizing operation was conducted under such conditions that theflow rate of the exhaust gas in the main flue 4 was 420,000 Nm /I-I; theflow rate in the flue 5 was 331,000 Nm /l-l; the flow rate in the flue 6was 89,000 Nm /H; and the temperature of the exhaust gas was 150C.

The exhaust gas passing in the branched flue 6 is used for theconcentration of the dilute sulfuric acid resulting from desulfurizationand the proportion of the exhaust gas flow rates in the flues 5 and 6 isadjusted such that the temperature of the joined exhaust gas at theinlets of respective activated carbon packed towers 14, 15 and 16 willbe about 100 150C. This temperature is variable depending upon theproperties of the activated carbon but is selected to be higher thanabout 100C. (50 80C. in the conventional apparatus) so as to facilitatethe dispersion of the desulfurized exhaust gas into the atmosphere froma chimney 118. The concentrations of the sulfur oxides, such as S0 inthe exhaust gas should of course be taken into account in determiningthe temperature.

From the view of the properties, namely the firing point of 300 400C.,the adsorption limit temperature of about 200C. and the durability, ofthe activated carbon commonly being used at the present time, thetemperature is selected within the range of about 100 150C. andpreferably within the range of 100 130C.

The exhaust gas branched into the flue 6 under the condition set forthabove is pressurized by a blower 7 and then passed through a highconcentration unit 9, a flue l and a low concentration unit 11, whilebeing cooled. The cooled exhaust gas flows in a flue 12 and is againjoined with the main flow of exhaust gas passing in the flue 5. Thetemperature of the joined exhaust gas is about 131C. Then, the exhaustgas is admitted into the activated carbon packed towers 14, 15 and 16through a flue 13 and exhausted into the atmosphere from the chimney 18at a temperature of 109C. It will be understood that the temperature ofthe exhaust gas being released into the atmosphere can be easilyadjusted by changing the mixing ratio of the gases passing in the flues12 and 5.

In the activated carbon packed towers the S0 adsorbed on the activatedcarbon is combined with O in the exhaust gas (through the exothermicoxidizing reaction) to form S0 which is thereafter desorbed in the formof dilute sulfuric acid by the washing water supplied through a pipe 21.The dilute sulfuric acid thus formed is collected in a washing tank 23,and thence delivered through a pipe 25 to a concentrating tank 26 or thetop of the low concentration unit 11 by means ofa pump 24. The dilutesulfuric acid in the concentrating tank 26 is sprayed from the top ofthe low concentration unit 11 through the pump 27 and a pipe 28, or isbrought into direct contact with the exhaust gas through the surface ofa filler, such as Rasching rings, to have the water present thereinevaporated, and then returned to the concentrating tank 26 through apipe 29, and this operation is repeated continuously.

The concentration of the dilute sulfuric acid in the tank 26 graduallybecomes high as the water therein is evaporated by the exhaust gas inthe manner described and the dilute sulfuric acid thus concentrated isdelivered into a tank 34 by a pump 30 and further concentrated by beingrepeatedly cycled through a pump 31, a pipe 32, a high concentrationunit 9 and a pipe 33 in the same manner as described above. As a result,sulfuric acid or high concentration is accumulated in the tank 34 anddelivered through a pipe 35 as the product sulfuric acid.

In the embodiment illustrated in the drawing, the temperature of thebranched exhaust gas is 150C. at the branched point, 95C. at the outletof the high concentration unit 9 and 60C. at the outlet of the lowconcentration unit 11. The exhaust gas thus cooled is led through a fluel2 and joined with the main flow of exhaust gas at the joint between theflues and 13. The concentration of the sulfuric acid in the tank 34 isabout 65 percent.

In desulfurizing the exhaust gas from a boiler in a thermal power plantor the like, according to the present invention, the dusts in theexhaust gas are removed by a dust collector, through not apparent in thedrawing. However, if the amount of the dusts is less than 30 50 mg/Nm,the apparatus of the invention is capable of treating the exhaust gaswithout necessitating the provision of a dust collector.

As described hereinabove, by employing the desulfurizing method andapparatus of the invention the concentration of dilute sulfuric acid,resulting from the water-washing of activated carbon in the desorptionstep, can be achieved by the effective use of the energy of the exhaustgas to be treated, without using a separate heat source, so that theapparatus is economical and of a great practical value as an industrialapparatus. It should be particularly noted that according to the presentinvention there is the advantage that the temperature of the exhaust gasbeing released into the atmosphere can be easily adjusted by regulatingthe flow rate of the exhaust gas branched from the main flow. Therefore,by elevating the temperature of the exhaust gas, it is possible toquickly disperse the exhaust gas in the atmosphere even under suchcondition that the atmospheric air is low in temperature, high inhumidity and steady in the morning and evening. With the conventionalapparatus, since the temperature of the exhaust gas released therefromis low, the exhaust gas under the condition described above stays abovethe ground surface without moving upwardly by being entrained in anascending air stream, thus causing a heavy damage to the agriculturalcrops, etc., and this has been the serious drawback of the conventionalapparatus. in this view, it will be understood that the method andapparatus of the invention are a great contribution to the prevention ofthe public damage.

We claim:

1. An exhaust gas desulfurizing apparatus for removing sulfur oxide froman exhaust flue gas containing sulfur oxide comprising a main flue gasduct for conducting a flue gas containing sulfur oxide from a flue gassource to a plurality of adsorption means packed with activated carbon,means for washing said adsorption means with a liquid to remove adsorbedsulfur oxide, means for collecting said washing liquid after it waswashed said activated carbon removing sulfur oxide adsorbed thereon,means connected to said collecting means for concentrating said washingliquid comprising a low concentrating means and a high concentratingmeans, a by-pass duct connected at first and second points along saidmain flue duct for conducting a portion of said flue gas from said fluesource through said high concentrating means and then through said lowconcentrating means and then to said main flue duct for conduction tosaid adsorption means.

2. An exhaust gas desulfurizing apparatus as defined in claim 1 whereinsaid concentrating means further includes a tank for each of said highconcentrating means and said low concentrating means, means forconnecting the concentrating tank to each of said concentrating means,and a pump for supplying the concentrated liquid in the concentratingtank for the low concentrating means to the concentrating tank for thehigh concentrating means.

3. An exhaust gas desulfurizing apparatus as defined in claim 1, whereinsaid low concentrating means is of the type in which the dilute washingliquid is concentrated by heat exchange with the exhaust gas in directcontact therewith.

4. A method of desulfurizing an exhaust gas containing sulfur oxidecomprising conducting a main exhaust gas flow of flue gas containingsulfur oxide from a flue gas source to an adsorption means packed withactivated carbon for adsorbing sulfur oxide from the gas, diverting aportion of said main flue gas flow, washing main flow to a level optimumfor the adsorption of the sulfur oxides by the activated carbon andconducting the joint flow to the adsorption means 5. An exhaust gasdesulfurizing method as defined in claim 4, wherein the proportion ofsaid branched exhaust gas and the remaining main flow of exhaust gas isselected such that the temperature of the joined flow of exhaust gaswill be in the range of C.

2. An exhaust gas desulfurizing apparatus as defined in claim 1 whereinsaid concentrating means further includes a tank for each of said highconcentrating means and said low concentrating means, means forconnecting the concentrating tank to each of said concentrating means,and a pump for supplying the concentrated liquid in the concentratingtank for the low concentrating means to the concentrating tank for thehigh concentrating means.
 3. An exhaust gas desulfurizing apparatus asdefined in claim 1, wherein said low concentrating means is of the typein which the dilute washing liquid is concentrated by heat exchange withthe exhaust gas in direct contact therewith.
 4. A method ofdesulfurizing an exhaust gas containing sulfur oxide comprisingconducting a main exhaust gas flow of flue gas containing sulfur oxidefrom a flue gas source to an adsorption means packed with activatedcarbon for adsorbing sulfur oxide from the gas, diverting a portion ofsaid main flue gas flow, washing the adsorption means with water toproduce dilute sulfuric acid, cooling the diverted flue gas with thedilute sulfuric acid resulting from the water washing of the adsorptionmeans, said cooling comprising conducting the diverted flue gas througha high concentrating means and then through a low concentrating meansthereby cooling the diverted flue gas by heat exchange, reintroducingthe cooled diverted flue gas into the main flow of exhaust gas to adjustthe temperture of said main flow to a level optimum for the adsorptionof the sulfur oxides by the activated carbon and conducting the jointflow to the adsorption means.
 5. An exhaust gas desulfurizing Method asdefined in claim 4, wherein the proportion of said branched exhaust gasand the remaining main flow of exhaust gas is selected such that thetemperature of the joined flow of exhaust gas will be in the range of100* - 150*C.